1. Technical Field
The present invention relates to wireless communication, and more particularly, to a method for allocating reference signals in a multiple-input multiple-output (MIMO) antenna system.
2. Related Art
A multiple-input multiple-output (MIMO) system is defined as a system that improves data communication efficiency by the use of multiple transmit antennas and multiple receive antennas. The MIMO system may be realized using a MIMO scheme such as a spatial multiplexing and a spatial diversity. According to the spatial multiplexing, different streams are concurrently transmitted through the multiple transmit antennas, and thus fast transmission is achieved without having to increase a system bandwidth. According to the spatial diversity, same streams are transmitted through the multiple transmit antennas to obtain diversity.
In order to reproduce a signal transmitted from a transmitter, channel estimation has to be carried out by a receiver. Channel estimation is defined as a process in which a distorted signal is restored by compensating for signal distortion due to fading. In general, for the channel estimation, reference signals are required which are known by both the transmitter and the receiver.
The reference signals may be allocated using either a first scheme in which the reference signals are allocated over the entire frequency band or a second scheme in which the reference signals are allocated over a part of the frequency band. The reference signals are further densely allocated in the first scheme rather than the second scheme. The channel estimation can be further accurately performed when the first scheme is used. On the other hand, a higher data rate can be achieved in the second scheme rather than the first scheme. In the second scheme, the reference signals are scarcely allocated, and thus the channel estimation may degrade.
In the MIMO system, multiple channels are independently provided for multiple antennas. The reference signals need to be allocated in consideration of the multiple channels. In addition, the MIMO system may operate in either a single-codeword mode or a multiple-codeword mode according to a rank. The number of reference signals may increase along with the increase in the number of transmit antennas. But, this may adversely affect the data rate.
Therefore, there is a need for a technique in which the reference signals can be effectively allocated in consideration of the multiple antennas.